A conventional telephony switching system, be it a public (e.g., central office) or a private (e.g., PBX) switching system, typically is able to fully recognize (e.g., validate) only telephone numbers that it serves, such as the extension numbers of its own, local, ports (commonly referred to as the switching system's internal dialing plan). If it receives a number that goes beyond identifying a local port, the switching system forwards the number on to another network entity, such as to another switching system or to a centralized network database, to which it is connected in the telephony network, for recognition of the number at the other entity. Thus, for example, a central office switch in a public network that follows the North American numbering plan recognizes dialed 7-to-10-digit numbers having the switch's own area code and office code. It sends out any dialed number having the same area code but a different valid office code at a port that corresponds to that office code, without recognizing the attached extension number. And it sends out any dialed number having a different area code at a port that corresponds to that area code, without recognizing the attached office code and extension number. Similarly, a PBX illustratively recognizes only 5-digit numbers having the switch's own prefix digit, sends out any 5-digit number having a different valid prefix digit at a port that corresponds to that prefix without recognizing the attached extension number, and sends out any 7-or 10-digit number at a port that is connected to the public telephony network without recognizing any of the numbers. Any switching systems that are connected to the ports of this call-originating switching system from which the non-recognized numbers were sent out may likewise recognize and respond to only a portion of the received number and may forward the number to yet another switching system or central database for ultimate recognition. In other words, the strategy employed in existing telephony systems is to break up a dialed telephone number into its constituent component segments and have a multiplicity of network entities in the telephony network recognize the components seriatim.
A consequence of this approach is that telephony network facilities--call-processing entities, switching-system ports, and inter-office trunks--are seized, occupied, and at least temporarily made unavailable for other use, while the dialed number is in the process of being recognized. Since it is typically unknown whether or not the call will succeed prior to the dialed number being recognized, the telephony network facilities are consumed irrespective of whether the call ultimately succeeds or fails. This means that the direct cost of using these facilities, as well as the cost of foregone alternative opportunities for use of these facilities, is incurred even if the dialed number is ultimately not recognized and the call fails. This is especially significant in the case of international calls, where the used facilities may span numerous switching systems, international boundaries and gateways, continents, oceans, and tens of thousands of miles of telephony trunks or satellite links.
Some PBXs take a small step toward alleviating this problem. For example, the AT&T System 75, 85, and Definity.RTM. PBXs include, within a single PBX, two call-processing arrangements, one (ARS) for the private-network numbering plan and another (AAR) for the public-network numbering plan. The private-network call-processing arrangement receives the dialed number first, and if it discovers that it is a public-network number (a 7-digit or a 10-digit number), it passes the number off for recognition to the public-network call-processing arrangement within the same switching system. Consequently, the use of one inter-office trunk, and of ports at each end of that trunk, is potentially avoided. In other relevant respects, however, the two call-processing arrangements function as described previously. So, while it has significant advantages not germane to this discussion, this approach does little to alleviate the problem described above. Also, this approach has the further disadvantage that it adds complexity and cost to the individual PBXs because it forces each PBX to include and support two separate call-processing arrangements.
Yet another approach, taken by "800"-number service arrangements and software-defined network (SDN) arrangements, involves collecting the entire number at an originating switching system and sending the entire collected number for full recognition to a central database that is shared by all of the switching systems. Clearly, this approach results in the use of network facilities external to the originating switching system--of a communications link interconnecting the originating switching system with the central database--prior to recognition of the dialed number and irrespective of whether the call will ultimately succeed or fail. Thus, this approach presents essentially the same disadvantage as the conventional telephony approach described above.